The present invention relates to electrically operated friction devices and controls therefor and more specifically to an electromagnetic self-adjusting clutch.
Electromagnetic clutch devices have been found to be particularly desirable in applications such as truck vehicles, in which the transmission of power to the cooling fan must be controlled by a device having considerable compactness. Such applications place particular requirements on the fan clutch in that it must be simple and economically constructed and at the same time provide a high torque output to enable the cooling fan to provide sufficient air flow across the engine thereby maintaining the engine's temperature below a predetermined limit. The clutch in such a system may be either off or on for long periods of time or cycled on and off for short periods of time depending on the output power and vehicle speed and, therefore, the clutch must be sufficiently durable to withstand constant use with a minimum amount of wear. Although known electrical magnetic clutches have proven to be successful in meeting the above requirements, the instant invention is concerned with an improvement in construction which enables operation of the clutch device without adverse wear effects on the armature and further, the invention provides an unlimited wear adjustment of the clutch surface without adversely affecting the force necessary to maintain the output torque.
Heretofore, there have existed at least two recognizable types of electromagnetic clutch constructions; one herein called the "cone" variation and the other herein called the "disc" variation. In the cone type interengaging friction surfaces between engageable rotatable clutch elements are conically shaped and by their inherent geometrical configuration require a lower axial force to develop sufficiently frictional locking of the working faces for rotation of the fan or driven member. In the disc type the interengaging friction surfaces are generally disposed normal to the axis of the engageable rotatable clutch element and in instances where the flux path passes through the working faces, the armature pull is entirely axial. The disc construction is particularly advantageous due to the flexibility for providing large axially directed flux path and thereby providing for a stronger clutch engaging force.
Other prior art designs are known which combine the desirable characteristics of the two known types of electromagnetic type constructions mentioned above. One such design provides an armature ring element which has a generally L shaped radial cross section; the element has one annular pole piece with a frustro-conical face and another annular pole piece with a flat disc-like face disposed normal to the clutch axis. This design, however, had several disadvantages. For example, since the armature ring element was one piece, there was no means for compensating for wear of either pole piece. Further, with the above-mentioned design, the conical friction surface must be made from a magnetic material. Yet another disadvantage results from this design in that the outer magnetic pole force is almost in the radial direction instead of the preferred axial direction.
Another design uses the same principle; threading the conical pole piece to the disc-like pole piece. This design added yet one more disadvantage to those listed above. By forcing the flux path to pass through the thread, there must necessarily be an even greater loss in the generated clutching force.
A further prior art design provides a conical frictional element which was moved into engagement with a mutually engageable conical face on an output member by the use of a resilient torque transmission member. The resilient torque transmission member provides a sufficient force to disengage the mutually engageable conical surfaces when the electromagnetic force becomes deenergized. Since the force of the resilient disengaging torque transmission member was in a direction opposite to the direction of the force generated by the electromagnetic flux path, out of necessity it required a greater electromagnetic force to engage the mutually engageable frictional torque transmitting faces. Therefore, as wear occurred, the electromagnetic force required to engage the frictional faces became of a higher magnitude since the resilient torque transmitting member had to be deflected a greater distance. Further adjusting for wear on such prior art device caused adverse wear on the armature face opposite the pole face. This was a result of the centrifugal force acting on the adjusting means. The centrifugal force caused the adjusting means to be somewhat delayed thereby causing the pole face of the output member to come in contact with the armature while rotating for a sufficient duration of time to cause adverse wear on the armature face.